The present invention relates to a process for making a lube base stock from materials having lower molecular weights.
Lubricant oils of high viscosity and high oxidation stability are desirable. Such materials can be prepared by hydrocracking, hydrodewaxing and hydrofinishing or another route of polymerization of normal alpha olefins (1-decene). The former route has the advantage of moderate costs, but the oxidation stability is not exceptional. As attempts are made to improve the oxidation stability by increasing the severity of the hydroprocessing steps, the yield of lube declines, as does its viscosity. The latter route gives an exceptionally stable product, but suffers the disadvantage of high cost. What is needed is a moderate cost process that generates high viscosity and highly stable products. The invention disclosed herein satisfied this objective.
The present invention provides a process for making a lube base stock from a lower molecular weight feedstock. In that process a highly paraffinic feedstock, with boiling points within the range of from 258xc2x0 to 1100xc2x0 F., is dehydrogenated in a dehydrogenation zone to produce an olefinic feedstock, which is oligomerized to produce an oligomerized product having a higher number average molecular weight than the olefinic feedstock. That oligomerized product is separated into a lighter fraction and a heavier fraction, the heavier fraction having a flash point within the lube base oil range. Preferably, either the oligomerized product is hydrogenated or the heavier fraction is hydrogenated, in both cases to eliminate any remaining olefins.
Preferably, the oligomerized product has at least 10% higher number average molecular weight than the initial feedstock, more preferably at least 20% higher than the initial feedstock.
Preferably, the highly paraffinic feedstock has a paraffin content of at least 75% prior to dehydrogenation. That highly paraffinic feedstock can be produced by a Fischer-Tropsch process, preferably hydrotreated prior to dehydrogenation. If feedstock is hydrotreated, skeleton isomerization can be induced during hydrotreatment.
Preferably, the olefinic feedstock has no more than 50% olefins, preferably from 10% to 50% olefins. Skeleton isomerization can be induced during that dehydrogenation step, or on the feed to the oligomerization step, or on the product from the oligomerization step.
Preferably, the heavier fraction of the oligomerized product has a viscosity of greater than 4 cSt at 40xc2x0 C. and a viscosity index of above 80 (more preferably above 120). Preferably, this heavier fraction is hydrogenated.
Preferably, at least a portion of the lighter fraction of the oligomerized product is recycled to the either to the dehydrogenation zone or to the first hydrotreatment zone. If the lighter fraction is recycled to the dehydrogenation zone, the oligomerized product is preferably hydrotreated prior to the separation step. If the lighter fraction is recycled to the first hydrotreatment zone, the heavier fraction is preferably hydrotreated.
In one embodiment, a lube base stock is made by
(a) using a Fischer-Tropsch process to produce a feedstock, with boiling points within the range of from 258xc2x0 to 1100xc2x0 F., and having a paraffin content of at least 75 weight %,
(b) hydrotreating and inducing skeleton isomerization in the highly paraffinic feedstock in a first hydrotreatment zone,
(c) dehyrogenating the hydrotreated highly paraffinic feedstock in a dehydrogenation zone to produce an olefinic feedstock having from 10% to 50% olefins, with the olefins being predominately internal olefins,
(d) oligomerizing the olefinic feedstock to produce an oligomerized product having a higher number average molecular weight than the olefinic feedstock,
(e) hydrotreating the oligomerized product in a second hydrotreatment zone;
(f) separating the oligomerized product into a lighter fraction and a heavier fraction, wherein the heavier has a viscosity of greater than 4 cSt at 40xc2x0 C., a viscosity index of above 120, and a flash point within the lube base oil range, and
(g) recycling the lighter fraction to said dehydrogenation zone.
In an alternative embodiment, the lighter fraction is recycled to the first hydrotreatment zone instead of to the dehydrogenation zone. In that embodiment, the hydrotreatment of the oligomerized product is not necessary. Instead, the heavier fraction is hydrotreated prior to hydrogenation.